Add FORCE/SKIP_BUNDLING flags for iOS builds

Summary:
*See discussion below for updated motivation.*

Anything running in Debug should use the packager anyway; no need to bundle. This saves a **huge amount of time** during development when testing things like push notifications that require a real device.

The code being modified was originally moved here in https://github.com/facebook/react-native/commit/9ae3714f4bebdd2bcab4d7fdbf23acebdc5ed2ba to make sure bundles are always created in `Release`, but the change can be applied to real devices, too. Ideally there should be very little difference in how a simulator is treated compared to a physical device.

Run a Debug build in Xcode targeting a physical device before and after this commit.

You can use the `FORCE_BUNDLING` and `SKIP_BUNDLING` flags to manually change the default behavior. For example, under **Build Phases** > **Bundle React Native code and images**:

```bash
export SKIP_BUNDLING=true
../node_modules/react-native/packager/react-native-xcode.sh
```
Closes https://github.com/facebook/react-native/pull/14731

Differential Revision: D5444352

Pulled By: javache

fbshipit-source-id: 68324fc0be7976e106fe0f9b31d763afd2b460a9